Both a sound volume and a sound quality are deteriorated in an earpiece of the mobile terminal device, or the like since leakage of sounds generated due to a clearance between the user's ear and the earpiece gives rise to a decrease in a sound pressure in a low-frequency band of the sound pressure-frequency characteristic.
In particular, the earpiece of the stationary terminal device has a relatively large shape and it is easy to cover the user's ear fully with this earpiece, but the earpiece of the mobile terminal device has a plane shape because such earpiece is formed integrally with other parts such as the display portion, and the like. Therefore, it is difficult to cover the user's ear fully with the earpiece and the leakage of sounds takes place more conspicuously, and thus a sound volume and a sound quality are deteriorated considerably.
As the reform measures for this, the measure for reducing the leakage by fitting a ring gasket to a contact portion between the earpiece and the user's ear to improve a sealing performance of the user's ear with the earpiece (see Patent Literature 1, for example), the measure for controlling a frequency characteristic by a signal processing means by fitting a pressure sensor near the earpiece to sense a contact state between the earpiece and the user's ear (see Patent Literature 2, for example), and the like have been proposed.    Patent Literature 1: JP-T-2002-501683    Patent Literature 2: JP-A-8-102991